Conventionally, there has been developed a semiconductor pressure sensor in which piezoresistive elements (strain gauge resistive elements) are respectively disposed at separate locations on a surface of a diaphragm to form a Wheatstone bridge circuit. The semiconductor pressure sensor detects deflection occurring in the diaphragm when a pressure is applied to the sensor by detecting a change in the output voltage of the Wheatstone bridge circuit due to the applied bias according to a change in the resistance of the piezoresistive elements.
In such a semiconductor pressure sensor, an offset voltage (output voltage of the Wheatstone bridge circuit when no pressure is applied to the sensor) varies when the power is supplied to the Wheatstone bridge circuit.
As a cause of the above, it is considered that movable ions that are present on the surface of the sensor are moved to the surface of the piezoresistive elements after the power is supplied, thereby changing the resistances of the piezoresistive elements. In view of the above, there has been proposed a method of suppressing the change in resistances of the piezoresistive elements due to the movable ions by providing a so-called electrical shield by forming a conductive film (shield thin film) on the surface of each of the piezoresistive elements through an insulating film and applying a predetermined potential, e.g., the lowest potential that can be applied to the Wheatstone bridge circuit, to the conductive film (see Patent Documents 1 and 2)    Patent Document 1: Japanese Patent Application Publication No. H8-86671    Patent Document 2: Japanese Patent Publication No. H2-41183
In Patent Document 1, each of the shield thin film is connected to the substrate potential via a connection film in a thick portion of the substrate, i.e., outside the diaphragm, and the shield thin films are controlled and maintained at the substrate potential to stabilize the resistances of the piezoresistive elements and suppress the drift of the output voltage.
Thus, since each of the shield thin films is connected to the substrate potential via a connection film outside the diaphragm, in order to avoid interference with the wiring for electrically connecting the piezoresistive elements, the layout area of the connection film increases. Accordingly, it leads to a problem such that the configuration of the sensor becomes large in size and the product cost increases. Further, it becomes necessary to provide the connection film for connecting the shield thin film to the substrate potential, and it is required to provide a manufacturing process for forming the connection film, thereby resulting in an increase in the number of manufacturing processes and the manufacturing cost.
In Patent Document 2, the shield thin films (shield metal film) respectively corresponding to piezoresistive elements are configured such that the shield potentials thereof can be arbitrarily chosen among the highest potential, an intermediate potential and the lowest potential of the Wheatstone bridge circuit. That is, all shield thin films are fixed to a common potential of any one of the above potentials.
However, in the piezoresistive element connected to the highest potential side of the Wheatstone bridge circuit and the piezoresistive element connected to the lowest potential side of the Wheatstone bridge circuit, a potential difference between both ends of the resistor, i.e., the voltage across the resistor itself is different. Therefore, when all shield thin films are fixed to the identical potential, a potential difference between the shield thin film and the piezoresistive element connected to the highest potential side is different from a potential difference between the shield thin film and the piezoresistive element connected to the lowest potential side. As a result, the degree of electrical influence on the piezoresistive element from the shield thin film may be different, and a variation may occur in the temperature characteristics and the resistance of the piezoresistive element, thereby deteriorating the offset voltage or offset drift.